1. Field of the Invention
The present invention is directed to underwater pressure compensation systems and, in certain particular aspects, to pressure compensation systems for closed-loop subsea hydraulic power systems.
2. Description of Related Art
The prior art discloses a wide variety of pressure compensation systems for underwater apparatuses and systems. In many underwater systems such as closed-loop subsea hydraulic power systems, it is desirable to maintain sufficient pressure within the system (an “overpressure”) to prevent the ingress of sea water into the system. In certain closed-loop systems, the fluid used in the system is re-circulated; but when subsea systems are at a substantial depth below the water surface designs that would withstand the pressure at such depths require inordinate and excessively strong enclosures. To overcome this problem, “pressure-compensated” systems have been developed in which a subsea equipment housing or enclosure need only withstand a pressure differential between the external pressure exerted on the enclosure by the water and an internal pressure which is maintained within the enclosure. In certain applications hydraulic fluid within an enclosure is pressurized by a spring that applies a force to a piston.
FIG. 1 illustrates schematically one typical prior art method for providing pressure compensation for hydraulic fluid F in an hydraulic fluid reservoir R which is in fluid communication with an apparatus A which is operated by the selective and controlled application of the hydraulic fluid F. A hollow body B has a piston P movably and sealingly mounted therein. The pressure of sea water S admitted through an opening O in the body B pushes against an outer face T of the piston P, pushing the piston P inwardly. Thus, the pressure of the sea water is applied to both the interior and to the exterior of the reservoir effecting the desired pressure compensation. A spring G biased between the piston face F and an inner wall W of the body B applies a force to the piston P, thereby providing additional pressure to the fluid F. Such systems work well if the volume of fluid F in the reservoir R is relatively constant with a maximum change on overall volume of 2-3 gallons or if the total overall volume is small, e.g. 2-3 gallons.
In the use of certain prior art subsea actuators, the actuator is not only remote from the hydraulic supply which is at the surface, but there can also be a substantial elevation difference. For example, with a pressure such as 3000 psi at the surface, the actual pressure at the actuator will be increased substantially beyond that by the weight or hydrostatic head of the fluid. The actual operating pressure of the accumulator is increased since the opposite side of the piston must discharge the hydraulic fluid either against the static head of a return line or against ambient seawater pressure, where water compatible hydraulic fluid is used. Seawater at a depth of 6700 feet has a static head of about 3000 psi. Accordingly, for an effective operating pressure of 3000 psi, the actual pressure at the actuator, and therefore at the accumulator is actually 6000 psi. A gas filled accumulator pressurized to 3000 psi at the surface would have the gas compressed to one half the volume at the operating depth and only half the hydraulic fluid would be available, while alternately the accumulator would have to be twice as large and, for an accumulator of the type which uses a compressed spring, this would require that the spring be compressed with an input force equivalent to 6000 psi initially. This becomes an exceedingly large and cumbersome mechanical spring system.
U.S. Pat. No. 3,987,708 discloses a system which uses a conventional gas charged accumulator with the high gas pressure providing the motive force for the accumulator and is depth compensated by means of a small hydraulic piston having one side open to the ambient, or sea pressure to provide depth compensation. This avoids the problem of the increased compression of the accumulator gas, but still requires that the accumulator be precharged to full gas pressure at the surface. It also contains extremely high pressure gas which must be sealed over a long period of time.
U.S. Pat. No. 4,777,800 discloses an hydraulic system accumulator designed to discharge its hydraulic capacity at a preselected pressure level, and designed to operate at a preselected depth, for instance, the known depth of a subsea wellhead. Charging of the accumulator at the surface is not required, the charge being developed as the accumulator is lowered to the desired depth. A piston assembly has a large diameter piston effectively exposed to the ambient pressure of the seawater and a small diameter piston effectively exposed to the hydraulic system pressure. The opposing side of each piston is exposed to contained low pressure gas. The differential area of the pistons causes the accumulator to buildup a predictable unbalanced force against the hydraulic fluid as a function of depth to which the accumulator is lowered.
There has long been a need, recognized by the present inventor, for an effective pressure compensation system for underwater systems and apparatuses. There has long been a need, recognized by the present inventor, for such systems for subsea hydraulic systems and for such hydraulic systems that are closed-loop and require relatively large amounts of hydraulic fluid to flow from a reservoir to operate equipment, and then be recirculated back to the reservoir.